The problem is that most of the time only one or at most two comps are able to establish the VPN connection successfully (not simultaneously) and the other machines can't establish the connection, as if the VPN server isn't out there, then after disconnecting, after few hours the same machine that had established the connection before now cannot establish it.

I've Netgear DSL router, i've a local RAS server and i've only one inbound entry in the router's firewall to that server, i've tried to delete that firewall role before and it fixed the problem, but temporarily